1. Field of the Invention
This invention relates to an improved line driver for ECL circuits and, more particularly, relates to a pulldown transistor arrangement for ECL output metal interconnect lines which functions independent of temperature and supply voltage variations.
2. Discussion of Background and Prior Art
Due to the exponential current-voltage characteristics and large transconductance of bipolar transistors, bipolar transistors are especially suitable for driving large capacitive loads. Although modern MOSFETs and bipolar transistors have approximately the same ring oscillator frequencies, it is the large driving capability of the bipolar transistors that accounts for the dominance of bipolar technology in high-speed VLSI applications. Because of its non-saturating nature and high functional capability, emitter-coupled logic has found widespread use in high performance circuits. In order to optimize overall circuit performance and take full advantage of bipolar devices over MOSFETs, it is important to enhance the driving capability of the ECL gates. For such ECL gates the low-to-high transition time is generally not a limitation because the output emitter followers are essentially not current limited and are capable of pulling the output line up to 1-2 ns. In high density integrated circuits high load capacitances of up to 1-10 pf may be experienced due to the cumulative length of all lines connected to the output, especially for large fan-outs. See, e.g., A. H. Dansky, "Halving Load Resistances Shortens Long Path Delays", Electronics, Oct. 9, 1980, p. 146. In a standard ECL gate, the output line is discharged via a constant pull-down current source during high-to-low transitions on the output line. Since the dc pulldown current is limited by power dissipation considerations, the high-to-low transition time on the output can be significant, especially when a heavy load is driven.
In the prior art the long high-to-low transition times described above have been tolerated or else active means have been utilized to pull down the output lines, i.e., to withdraw the charge held on the parasitic capacitance of the interconnect line. For example, in J. E. Price, "Emitter-Coupled Logic Circuit", U.S. Pat. No. 4,347,446, an active pulldown arrangement is proposed which comes into effect when high-to-low transitions are taking place. This approach requires a separate power supply and does not function in a manner which is independent of temperature variations or operate independent of supply voltage.
It is an object of the present invention to provide an active pulldown means for the output line of an ECL gate which operates over the full military temperature range independently of changes in temperature.
It is another object of the present invention to supply an ECL compatible line driver which does not require a separate power supply and operates independent of variations in the existing supply voltage.